This invention relates to devices and methods for compressing a self-supporting web, as for example to reduce the caliper of a web, to remove fluid from the web, or to laminate the web.
In the processing of self-supporting webs such as paper it is often necessary to compress the web. For example, calendar rolls may be used to reduce the thickness or caliper of a web and pinch rolls or nip rolls may be used to squeeze liquid out of the web. Pinch rolls have also been used to press two adjacent layers together to produce a laminate.
Such pinch or nip rolls are made up essentially of two opposed rolls which rotate in opposite directions and between which the self-supporting web to be processed is passed. Because both of the opposed rolls are cylindrical in cross-sectional shape, each portion of the self-supporting web is subjected to a short, high pressure spike as it moves between the rolls. Because the high pressure is applied to the web for only a relatively short time, it is not uncommon for a processing operation to require multiple calendar or nip rolls acting on the web in sequence.
In the past, pulp suspensions have been washed in devices including converging chambers. Ingermarsson, U.S. Pat. No. 3,616,660 and Burling, U.S. Pat. No. 3,564,631 describe two examples of such devices. In both cases a slurry of fibrous pulp and water is introduced into a converging chamber having porous walls. As the slurry moves through the chamber, the pulp remains in the chamber and a portion of the liquid in the pulp passes through the porous walls of the chamber. Once the liquid in the pulp has been removed, additional liquid is then used to wash the pulp. Though Ingermarsson and Burling employ converging chambers, neither is suitable for use with a self-supporting web. In both cases, conduits are used to supply the pulp bearing slurry to the processing apparatus.
Recently, a pressure saturator has been developed which employs a converging chamber to force a saturant into a substrate. This pressure saturator is described in U.S. Pat. No. 4,588,616, which is assigned to the assignee of this invention. As described in this patent application, the pressure saturator introduces the pressurized saturant into the substrate. This is quite a different operation from that contemplated by the present invention, which involves operations such as reduction of the caliper of the web, drying of the web, or lamination of a multilayer web.